Exhaust gas restrictions have become increasingly severe in recent years, and among restrictions in place for HC, CO and NOx, steps have been taken to tighten regulations on NO in particular. NOx storage reduction catalysts, obtained by loading an NOx storage material such as an alkaline metal, alkaline earth metal or rare earth element on carrier particles along with a precious metal, are being used to improve NOx storage performance in exhaust gas purification catalysts.
NOx storage reduction catalysts are known in which an NOx storage material such as an alkaline metal or alkaline earth metal is loaded on carrier particles together with a platinum group metal such as platinum (Pt), palladium (Pd) or rhodium (Rh). Among these, Pt and Pd mainly contribute to oxidative purification of CO and HC and oxidation of NO to NO2 in lean atmospheres, while Rh mainly contributes to reductive purification of NO2 in rich atmospheres. The alkaline earth metal Ba temporarily stores NO2 formed as a result of oxidation by Pt and Pd in lean atmospheres, and then releases the storage NO2 in stoichiometric to rich atmospheres. The released NO2 is then purified by reacting with a reducing component contained in exhaust gas due to the action of Rh.
With respect thereto, Patent Document 1 discloses an NOx storage reduction catalyst obtained by loading Pt and Rh on the same carrier particles followed by further loading Ba to have Pt, Rh and Ba present on the same carrier particles.
Patent Documents 2 and 3 disclose that oxygen poisoning of Rh due to the effects of Ba is promoted and NOx purification performance decreases in the case Rh and Ba are both present on the same carrier particles. With respect thereto, Patent Documents 2 and 3 report NOx purification performance is improved by respectively loading Rh and Ba on different carrier particles in a mutually separated state.